Grazing incidence X-ray diffraction of a photoaligned nematic semiconductor.

Grazing incidence X-ray diffraction is used to find the thin film morphology of an extended molecule with an irregular alternating fluorene-thiophene structure, which is used to obtain linearly polarized electroluminescence and the photovoltaic effect. The material has a room temperature nematic glassy phase and is uniaxially aligned in the plane of the film using photoalignment techniques. Two distinct intermolecular separations of 0.45 and 1.5 nm are identified showing that the molecules are lamellar. The lamellae stack with only local order and the two short axes of the lamellae have no preferred orientation at the surface or bulk of the film. Neighboring molecules show a wide range of longitudinal displacements along the axis of the director, as expected for a nematic liquid crystal with no positional order. There is, however, a dominant feature corresponding to a longitudinal offset of 0.51 nm. Unlike some other fluorene-containing semiconductors where microphase separation of the side chains inhibits close packing of neighboring molecules, the lamellar structure and 0.45 intermolecular spacing found here allows pi-pi intermolecular interactions for efficient carrier transport. We obtain a room temperature hole mobility up to 3.4 x 10-3 cm2 V-1 s-1 using a time-of-flight technique.

Triplets in extended nematic liquid crystals and polarons in their blends.

Photoinduced absorption shows that triplets are the primary photoexcited species in a series of conjugated liquid crystals containing thiophene and fluorene groups. We find that the triplet generation rate can be varied substantially by molecular design. The introduction of extra thiophene groups into the elongated molecules changes the intersystem crossing rate by over two orders of magnitude, while modifying the singlet and triplet energies by only small amounts. This result is attributed to the high spin-orbit coupling constant of sulfur: An increase in the number of sulfur atoms increases the spin-orbit coupling between the singlet and triplet states. These results are relevant to the design of organic light emitting diodes, lasers, and other devices where triplet formation has a major impact on device performance. The molecules are shown to act as effective electron donors when blended with a perylene molecule which acts as an electron acceptor. The electron transfer rate is faster than the singlet lifetime so that the blend shows the efficient charge separation required for a photovoltaic device.

Charge-transport in crystalline organic semiconductors with liquid crystalline order.

We report the design and synthesis of a liquid crystalline material exhibiting highly ordered smectic phases and high charge carrier-mobility; by a process known as "paramorphosis" highly ordered smectic phases can be transferred to the amorphous crystalline state on crystallisation without the formation of significant crystal grain boundaries and deep traps.

Electron-transporting and photopolymerisable liquid crystals.

The synthesis of a novel, photopolymerisable liquid crystal (reactive mesogen) with a high mobility of electrons in the smectic C phase at room temperature is reported for the first time as a potential charge transport layer for OLEDs.